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Abilities: Drop

The drop ability allows an instance of a struct to be ignored or discarded. In many programming languages, this behavior is the default. However, in Move, a struct without the drop ability cannot be ignored. This is a safety feature that ensures all assets are properly handled. Ignoring a struct without the drop ability results in a compilation error.

module book::drop_ability {

    /// This struct has the `drop` ability.
    public struct IgnoreMe has drop {
        a: u8,
        b: u8,
    }

    /// This struct does not have the `drop` ability.
    public struct NoDrop {}

    #[test]
    fun test_ignore() {
        let no_drop = NoDrop {};
        let _ = IgnoreMe { a: 1, b: 2 }; // no need to unpack

        // The value must be unpacked for the code to compile.
        let NoDrop {} = no_drop; // OK
    }
}

The drop ability is often used in custom collection types to eliminate the need for special handling when the collection is no longer needed. For example, the vector type has the drop ability, allowing it to be ignored when no longer needed. However, the most significant feature of Move's type system is the ability to not have drop, ensuring assets are properly handled and not ignored.

A struct with only the drop ability is called a Witness. The concept of a Witness is explained in the Witness and Abstract Implementation section.

Types with the drop Ability​

All native types in Move have the drop ability, including:

All types defined in the standard library also have the drop ability, such as: